IEEE 802.11ac supported MU transmission using spatial degrees of freedom via MU multiple-input multiple-output (MIMO) in the downlink (DL) direction from the access point (AP) to the stations (STAs). To improve efficiency, IEEE 802.11ax is considering using both Orthogonal Frequency Division Multiple Access (OFDMA)/MU-MIMO in both DL and UL directions. In addition to supporting frequency and spatial multiplexing from AP to multiple STAs, transmissions from multiple STAs to the AP are also supported, i.e., MU transmissions.